Chromium base alloy



Jan. 4, 1966 J. w. CLARK 3,227,548

CHROMIUM BASE ALLOY Filed Feb. 18, 196s United States Patent O 3,227,548CHUR/HUM BASE ALLY Kael; W. Clark, Milford, hio, assignorto GeneralElectric Company, a corporation of New York Filed'Feh. 18, 1963, Ser.No. 258,993 6 Claims. (Cl. 7S-176) This invention relates to chromiumbase alloys and, more particularly, to chromium base alloys strengthenedby a combination of carbide dispersion strengthening and solutionstrengthening.

In co-pending application Serial No. 179,941, filed March 15, 1962 andassigned to the assignee of the present invention, there is described animproved chromium base alloy strengthened by the dispersion of carbidesformed from carbon and Group IV A elements of the Periodic Table ofElements. Although the alloys of that copending application achieve goodstrength and oxidation characteristics at elevated temperatures, it isnecessary for use in aircraft or rocket propulsion apparatus thatchromium base alloys be developed which have the high 3,227,543 PatentedJain. 4, 1.956

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alloy. However, it has been found that the addition to the melt of morethan about 1.5 weight percent Y results in alloy brittleness(hot-shortness) at elevated temperatures whereas the addition of lessthan about 0.5 weight percent Y results in the retention of too littleyttrium in the alloy for improvement in an oxidation resistance aftermelting.

It has been recognized that a combination of solution strengthening bythe addition of tungsten and the dispersion strengthening byprecipitation of a complex carbide system is very effective inincreasing the elevated temperature mechanical properties of chromium.It has been discovered that the careful selection of amounts of solutionstrengthening and dispersion strengthening additions at the same timecan result in alloys which have a very desirable combination offabricability, elevated-temperature strength, low-temperature ductilityand resistance to air oxidation.

Typical of example alloys which were studied in the evaluation of thepresent invention are those included in the following Table l.

TAB LE I Example Weight percent (balance Cr) Other Ti-l-Zr-l-Hf Zr Ti CY (added) Y (retained) NA-Not available.

temperature characteristics at least equal to the best available nickelbase super-alloys.

Therefore, it is an object of this invention to provide a chromium basealloy'having high strength characteristics and oxidation resistance atelevated temperatures while at the same time being suiciently workableto allow its fabrication into useful articles.

This and other objects and advautags will he more readily recognizedfrom the following detailed description and examples, all of which aremenat to be typical of, but not limitations on, the scope of theinvention.

The accompanying drawing is a graphical representation of the stressrupture properties of the alloy of this invention compared with otheralloys.

Briefly, the improved high strength chromium base alloy of the presentinvention consists of, by Weight, -15 W; up to about 10% Mo; about(lA-2% of elements selected from the group consisting of Ti, Zr and Hf;0.05- 0.2% C; up to about 0.7% retained Y; with the balance chromium andincidental impurities.

In a preferred form, the alloy of the present invention consistsessentially of, by weight, 5-15% W, 0.4-1.2% Zr, up to 0.4% i, 0.05-0.2%C, upto about 0.7% retained Y, with the balance Cr and incidentalimpurities.

T he advantages of additions of yttrium and its gettering eiect inreducig the oxygen content of alloy melts, as well as minimizing theoxidation rates and absorption `of nitrogen during their working or usein air have been described in the above-identitied co-pendingapplication as well as in the United States'Patent 2,955,937-McGurty etal. The present invention employs yttrium (1) as a gettering agent and(2) for its value when retained in the TABLE Il Vic-kers Hardness(kg/mini) Example Cast Extruded Swagcd B Extensive solidieation cracksin ingot.. b Too high hardness to fabricate.

Alloys M 197, M 198 and M 199 in Table Il define the practical limits ofthe present alloy from a fabricability point of view. Although the 800to 1200 ton extrusion presses employed as ingot breakdown equipment wereunable to process alloys M197, M 198 and M 199, they should be amenableto processing by higher capacity presses. The high strength potential ofsuch `alloys are indicated by the Vickers hardnesses of from 325 to 374lig/mm?. lt has been found that inclusion of more than about 15 weightpercent tungsten or the combination of tungsten and molybdenum providestoo much solution strengthening for good fabricability. Similarly, asshown by M 197, a carbon level of as high as 0.2% with a lower tungstenlevel results in too much precipitation strengthening for good ingotsoundness and fabricability. Thus, with regard to W and C, the twoprincipal elements involved in solution strengthening and precipitationstrengthening it has been found that the tungsten level should bemaintained at about weight percent or less and the carbon level shouldbe maintained at about 0.2 weight percent or less.

With regard to M 179 which had extensive solidication cracks in theingot due to the high Ti level as well as M 197 which had high hardnessdue to the high carbon and Ti level, it has been found that a titaniumlevel of greater than about 0.4 weight percent not only results inadverse fabricability characteristics, but also, as shown in Table IIIbelow, decreases the oxidation resistance of the alloy.

The primary carbide formers of the alloy of the present invention arethe elements Hf, Zr and Ti. It has been recognized that the total ofthese elements in the alloy of the present invention should bemaintained in the range of about 0.4-2 weight percent. The preferredcarbide obtained in the present invention is predominantly themonocarbide ZrC. However, the element titanium can be added to the alloyof the present invention up to about 0.4 weight percent to replace someof the zirconium in the monocarbide thereby to increase the stability ofthe dispersed phase or phases in the alloy. As was mentioned above, theaddition of too great an amount of titanium will result in undesirablecharacteristics.

A comparison of examples M 180, M 184 and M 201 in Tables I and II showsthat Hf can be substituted for Zr or Ti as a carbide former. It is to benoted that on an ato-mic basis the metal to carbon ratios in Examples M180, M 184 and M 201 are the same. In Example M 201, titanium has beeneliminated because in some applications Ti is slightly harmful from anoxidation point of view. Thus, the same type of strengthening throughcomplexing of carbides is accomplished by simultaneous addition of Hfand Zr without sacrice of the excellent oxidation resistance of M 184,as shown in Table III. The improvement in cast hardness of M 201 over M184 shown in Table II suggests that strength improvements would resultand limited stress-rupture data presented in Table V shows this to bethe case.

TABLE IIL-AIR OXIDATION RESISTANCE AT 2000 F, 24 HOURS Alloy Weight gain(mg/cm?) Deptlrhltllrcened l Depth below surface scale(s) hardened byinternal reaction with Nl and/or O9.

The following rI'ables IV `and V show tensile and stressruptureproperties of some of the alloys of Table I.

TAB LE IV.-TENSILE P ROPE RTIES Ultimate (ksi.) 0.2% yield (k.s.i.)Percent elongation Example i 1800 2000 2200 1800D 2000D 2200 1800 20002200 F. F. F. F. F. F. F.

TABLE V.-STRESS RUPTURE PROPERTIES AT 2000 F.

Example Stress (k.s.i.) Life (hrs.)

aTest discontinued, no failure.

Referring to Table IV, it is easily seen that Examples M 180 and M 187are the strongest of the alloy forms in the series. This is furthershown in Table V with regard to stress rupture properties.

In Table IV the comparison of M 132 and M 183 shows the striking effectof the addition of l0 weight percent tungsten to a Cr-Y alloy forsolution strengthening purposes. At the same time, comparison of M 188,M 178 and M 180 shows that increasing the tungsten from 0 to 5 to 10weight percent, in combination with carbon and the proper amounts ofstrong carbide forming elements, results in a further and significantstrength increase. Note the unusual and unexpectedly great differencebetween the stress rupture properties of M 180 and M 178 in Table V.Thus, in example M180, the inclusion of the proper combination ofsolution strengthening and precipitation strengthening elements resultsin an unusually high strength chromium base alloy which is fabricableand which has adequate oxidation resistance. Useful low-temperatureductility of the alloy of this invention can be attained by propercontrol of processing variables. Tensile elongation lof 5 to 10% aremeasured at 75 F.

The drawing is a graphical representation combining a large amount ofdata and comparing the stress-rupture properties of the preferred formof the alloy of this invention, M 180, with other alloys such as M 178and alloys M 188, M 132 and chromium which lie outside `of the scope ofthe present invention. It can be seen from this data that thestress-rupture strength of alloy M 180 is significantly greater thanother alloy forms. Although the hour points in the drawing have beenindicated, the horizontal coordinate has been represented in the form ofthe Larson-Miller parameter formula P=T (20 -llog t) in which P is theLarson-Miller parameter number, T is the temperature -in degree Rankineand t is the time in hours. It is well established in the metallurgicalart that the Larson-Miller parameter plots can be used, once the curvesare established by actual data, to compare alloys along any point on thecurves.

All the alloys of the tables and those represented in the drawing wereinduction melted as 8 to 12 pound ingots in a 20 kilowatt inductionfurnace. The chromium and tungsten charges were pressed into briquettesand heated to about 1800 F. in vacuum. Argon or helium was thenintroduced and the charge was melted. After holding the melt in a liquidstate, the remaining alloy additions of Ti, Zr, Hf, Y or C were charged.The melt Was held for a short period to promote homogenization and thenwas cast. Initial breakdown of the cast structure Was accomplished byextrusion at temperatures between 2400 and 2800 F.

In order tok accomplish elevated temperature tensile and creep rupturetests, button-head speciments were ground from swaged stock to a gagediameter of 0.160" and a gage length of 1.1. High temperature tensiletesting was performed in a vacuum of less than about using a lfcrossheadspeed of about 0.01 per minute. The constant-load creep rupture testswere conducted -in vacnum purged capsules, back-iilled with helium at aslight positive pressure.

Although the present invention has been described in connection withspecific examples, it will be recognized by metallurgists the variationsin modifications of which this invention is capable without departingfrom its eiective scope.

What is claimed is:

1. An improved chromium base alloy consisting of, by Weight: 5-15% W; upto about 10% Mo; about 0.4-2% of elements selected from the groupconsisting `of Ti, Zr and Hf; 0.05-0.2% C; up to about 0.7% retained Y;with the balance Cr and incidental impurities.

2. An improved chromium base alloy consisting of, by weight: 5-l5% W;0.4-1.2% Zr; up to 0.4% Ti; 0.05- 0.2% C; up t-o about 0.7% Y; with thebalance Cr and incidential impurities.

3. An improved chromium base alloy consisting of, by Weight: about 10%W; up to about 5% Mo; about 0.4 2% of elements selected from the groupconsisting of Ti, Zr and Hf; 0.05-0.2% C; up to about 0.7% retained Y;with the balance Cr and -incidental impurities.

4. A high strength chromium base alloy consisting of, by weight: about10% W; 0.4%-0.8% Zr; (X1-0.2% Ti; 0.05-0.1% C; up to about 0.7% Y withthe balance Cr and incidential impurities.

S. A highly oxidation-resistant chromium base alloy consisting of, byweight: about 10% W; 0.6-1.2% Zr; 0.05 %-0.1% C; up to about 0.7% Y withthe balance Cr and incidental impurities.

6. A strong, oxidation-resistant chromium base alloy consisting of, byWeight: about 10% W; 0.6-1.2% Hf; 0.3-0.6% Zr; 0.05-0.1% C; up to about0.7% Y with the balance Cr and incidental impurities.

References Cited by the Examiner UNITED STATES PATENTS 2,780,545 2/1957Blank et al 75-176 2,955,937 10/ 1960 McGurty et al 75-176 3,017,2651/1962 McGurty et al 75-176 3,027,252 3/ 1962 MCGurty et al. 75--176DAVID L. RECK, Primary Examiner.

1. AN IMPROVED CHROMIUM BASE ALLOY CONSISTING OF, BY WEIGHT: 5-15% W; UPTO ABOUT 10% MO; ABOUT 0.4-2% OF ELEMENTS SELECTED FROM THE GROUPCONSISTING OF TI,ZR AND HF; 0.05-0.2% C; UP TO ABOUT 0.7% RETAINED Y;WITH THE BALANCE CR AND INCIDENTAL IMPURITIES.